978-1-4244-4522-6/09/$25.00 ©2009 IEEE ISCIT 2009 1173 An Inter-Carrier Interference Reduction Scheme for OFDM Underwater Acoustic Communications L F Yeung 1 Geng Nian 2 Zhan Choujun 3 Department of Electronic Engineering, City University of Hong Kong, Hong Kong SAR eelyeung@cityu.edu.hk 1 gnian2@student.cityu.edu.hk 2 Abstract— The inter-carrier interference (ICI) of the orthogonal frequency division multiplexing (OFDM) symbols caused by the transducers dynamic in underwater acoustic (UWA) communications has been presented in this paper. Both the practical experiment and simulation results show that the othorgonality property of OFDM symbols can be affected by the transducers dynamic during the transient period of these devices especially when the OFDM symbol length is comparatively short. A small cyclic prefix (CP) method is proposed to overcome this problem without much loss of the throughput and the bandwidth efficiency. The small CP, with the length just longer than the transient response of the transducer, can eliminate the ICI problem. The theoretical analysis and the simulation results show the efficiency of the method. Keyword: Underwater acoustic communications, OFDM, inter-carrier interference, effects of transducers dynamic. I. INTRODUCTION Underwater acoustic (UWA) communications over shadow water and long distance channels are difficult due to oceanic variables like large Doppler, time spread, salinity variation, differential temperature layers, ocean currents, frequency shift, amplitude variations and the multipath fading problems [1]. Non-coherent multiple-frequency-shift-keying technique is a robust modulation technique used extensively in UWA communications, especially in rapidly time-varying, phase variation and Doppler shift environments such as the shallow water long and medium-range channel [1]. Modified frequency modulation (MFM) is a relatively robust scheme for UWA communication by converting a data stream into a set of N parallel digitally modulated carriers at frequencies {f i } with narrow frequency spacing Δf. Data can be transmitted in parallel at a lower data rate with longer symbol time. The trade-off is the reduction of data throughput compared with the coherent type techniques. Frequency diversity can be added on top of this frequency multiplex operation in a form of spreading code like code division multiple access (CDMA) scheme to solve the frequency selective fading problem. Alternatively, orthogonal frequency division multiplexing (OFDM) provides closer inter- subcarrier spacing and it is a band efficient multi-tone modulation technique widely used in wireless communications. The OFDM technique is a common and reliable implementation, where the inter-symbol interference (ISI) elimination can be achieved by a simple cyclic prefix (CP) insertion and removal technique [3]. Many standard core, modules and libraries are available for easy embedding applications [4]. Recently, OFDM has been widely used in UWA communication for its simple structure and bandwidth efficiency that are crucial in UWA communication systems [1] [3] [5]. Especially, if the CP length is longer than the maximum time-delay of the UWA channel the serious ISI problem in UWA communication systems can be eliminated but with around 25% or more increase of the symbol length [4][7]. Many communication systems apply none CP technique, which is named as zero padding (ZP), and use channel identification and equalization technique to solve the ISI problem [8][9]. In this paper we also adopt the same strategy with almost no CP and assume the ISI problem can be eliminated using equalization. However if the CP is omitted, th e orthogonality of the pure OFDM symbols will be destroyed by the dynamic effects of the transducers even before the signal is transmitted in the real underwater channel. This phenomenon has been observed from both practical experiment and computer simulation. Then the small CP approach has been proposed to eliminate the ISI and inter- carrier (ICI) caused by the transducer dynamic. The theoretical analysis and simulation results also show the efficiency of this small CP strategy to overcome the. II. TRANSDUCERS DYNAMIC A. System Model of Transducers An electro-acoustic transducer system like hydrophone and transducer transmits information through a fluid medium, which is similar to antennas transmitting the information through the electromagnetic field medium. But transducers are electro-mechanical-acoustic devices with complicated dynamics, which can be generally represented by non-linear transfer functions. Many efficient piezoelectric electro- acoustic energy conversion devices are high-Q and strong resonance devices with nature resonance frequency ω n . The typical electrical characteristic of transducers is shown in Fig.1 (a). In our experiment, the same type of transducers is used as transmitter and receiver. Here we assume that transducers are operated in the linear range and the transfer function of the transducers combination is H(s) = H t (s) · H r (s), (1) where H t (s) and H r (s) are the transfer functions of the transducer and the hydrophone, respectively. All parameters of H t (s) and H r (s) are obtained via measurement. The impulse response of the transducers denoted by h(t) is shown in Fig.1(b). The settling time is around 0.6ms. ________________________________________________ This work is supported by CERG Grant No. CityU 1339/04E